When hydrocarbon producing wells are drilled, initial hydrocarbon production is usually attained by natural drive mechanisms (water drive, solution gas, or gas cap, e.g.) which force the hydrocarbons into the producing wellbores. If a hydrocarbon reservoir lacks sufficient pore pressure (as imparted by natural drive), to allow natural pressure-driven production, artificial lift methods (pump or gas lift, e.g.) are used to produce the hydrocarbon.
As a large part of the reservoir energy may be spent during the initial (or "primary") production, it is frequently necessary to use secondary hydrocarbon production methods to produce the large quantities of hydrocarbons remaining in the reservoir. Waterflooding is a widespread technique for recovering additional hydrocarbon and usually involves an entire oil or gas field. Water is injected through certain injection wells selected based on a desired flood pattern and on lithology and geological deposition of the pay interval. Displaced oil is then produced into producing wells in the field.
Advancements in secondary hydrocarbon producing technology has led to several improvements in waterflood techniques. For example, the viscosity of the injected water can be increased using certain polymer viscosifiers (such as polyacrylamides, polysaccharides, and biopolymers) to improve the "sweep efficiency" of the injected fluid. This results in greater displacement of hydrocarbon from the reservoir.
Ability to displace oil from all the producing intervals in a hydrocarbon reservoir is limited by the lithological stratification of the reservoir. That is, there are variations in permeability which allow the higher permeability zones to be swept with injected fluid first and leave a major part of the hydrocarbon saturation in the lower permeability intervals in place. Continued injection of flooding fluid results in "breakthrough" at the producing wells at the high permeability intervals which renders continued injection of the flooding medium uneconomical.
Profile control has been used to prevent or correct "breakthrough" at high permeability intervals. Profile control involves using stable polymers to retard or, in some cases, block off the higher permeability intervals in a mature flood so that the flooding media is diverted to the lower permeability intervals. Field experience has indicated profile control can be used to enhance productivity from lower permeability intervals.
To impart profile control, specially developed polymers (such as copolymers and polysaccharides) are used to retard permeability of higher permeability intervals. However, permeability damage is imparted to the lower permeability zone while the profile control material is being used to treat the high permeability interval(s).
Although fluid entry into the low permeability zone is retarded by the profile control polymer, laboratory tests indicate a filter cake build-up at the formation interface and a limited depth of damage.
Therefore, what is needed is a profile control method wherein a high permeability zone can be closed to fluid while minimizing damage to a low permeability zone.